[Ada] Restrict initialization of External_Tag and Expanded_Name

[Ada] Restrict initialization of External_Tag and Expanded_Name

2018-05-23  Javier Miranda  <miranda@adacore.com>

gcc/ada/

	* exp_disp.adb (Make_DT): Restrict the initialization of
	External_Tag and Expanded_Name to an empty string to the case where
	both pragmas apply (i.e. No_Tagged_Streams and Discard_Names), since
	restricted runtimes are compiled with pragma Discard_Names.
	* doc/gnat_rm/implementation_defined_pragmas.rst,
	doc/gnat_rm/implementation_defined_characteristics.rst: Add
	documentation.
	* gnat_rm.texi: Regenerate.

From-SVN: r260584